The invention relates to a slag and corrosion control process, which is especially useful in the control of both slag and chloride corrosion in boilers, particularly waste to energy (WTE) and biomass boilers. Super heater tubes in WTE boilers are particularly susceptible to slagging and corrosion. Economical solutions to these problems are essential if this technology is to reach its fullest potential to the environment and national and enterprise energy security.
In the United States, WTE boilers have been built and operated primarily for environmental reasons. It would be desirable to improve their economics so that economic incentives would be added to environmental concerns.
WTE boilers burn solid waste, e.g., refuse derived fuels (RDF) made from solid municipal waste, and eliminate the need for large tracts of land to be set aside for solid waste disposal. They partially offset capital and operating costs by generating electrical power and/or steam energy. It would be desirable from the standpoint of the environment and energy security to foster their wider use. Because economics will always play a significant role in the choice between WTE plants and landfills, it is important to find ways to reduce costs for WTE plants.
The generation of solid waste follows population density, which places economic pressure on landfill availability and costs. The economics of WTE plants is a complex matter. As it stands today, WTE installations are economically competitive with landfills in more populated areas but have not seen widespread use in areas where land is still plentiful. The selection of a WTE plant instead of landfill is dependent on many cost factors including the cost of landfill operations, the cost of transporting waste to the landfill, the cost of generating electricity by burning fossil fuels, the cost of WTE plant installation, the cost of WTE operation, and many other costs including those for maintenance and repair of the WTE plants. Where the maintenance costs can be decreased, the installation and use of WTE plants and their positive benefit to the environment will be increased because improved economics will foster their wider usage.
The current operational and maintenance costs associated with slagging and, especially, corrosion in WTE boilers create economic burdens now, but their efficient control offers significant savings. The problem of corrosion in WTE plants is so severe that it has deterred their more widespread adoption for economic reasons. Better heat recovery could be achieved by reducing slagging, and boilers could be run more regularly if slagging could be reduced and/or made easier to remove. Both better heat recovery and more regular operation will improve the economics of WTE plants versus power plants burning fossil fuels. Improving economics will provide an eventual increase in the use of WTE and will also provide benefits in energy security because fossil fuels will be replaced to at least some significant degree by using RDF, which is projected to remain available in vast quantities.
Because the problems of slagging and corrosion are so large for WTE boilers, solution of these problems offer great incentives for more widespread use of this technology. The RDF as typically burned tends to be highly corrosive and to promote slagging. Because plastics and other materials of commerce that find their way into municipal solid waste can contain more halogens, e.g., chlorides, than typical fossil fuels, the problem of corrosion for WTE facilities has grown in importance as these plants take on greater environmental and economic importance. Slagging reduces heat transfer from the combustion gases to water held in heat exchangers, and removing slag from heat exchange surfaces is costly. Slag removal adds two-fold costs: it typically requires shutting the facility down for cleaning, and the materials and manpower required for cleaning add further costs. There is a need to address both corrosion and slag control with new technology that can increase the overall economics of WTE boilers and foster their use for improving the environment and contributing to national energy security.
Slagging deposits are sometimes extremely difficult to remove by conventional techniques such as soot blowing. Slag buildup results in a loss of heat transfer throughout the system, increases draft loss, limits gas throughput and is a factor in tube failure due to erosion from excessive soot blowing. A variety of procedures are known for adding treatment chemicals to the fuel or into the furnace in quantities sufficient to treat all of the ash produced, in the hope of solving the slagging problem. More recent technology provides for targeted chemical introduction where the chemical is directed at trouble spots in a boiler. Typical slag control chemicals include magnesium oxide and magnesium hydroxide. See, in this regard, U.S. Pat. Nos. 5,740,745 and 5,894,806 and U.S. Patent Publication No. 20050150441 and the references cited therein. While these chemicals can reduce the severity of slagging and make cleaning significantly easier, they have not been developed to the extent desired to fully achieve the economic and environmental advantages of WTE facilities. It would be desirable, for example, to so reduce the severity of slagging that the reduced application of mechanical energy could effectively clean heat exchange surfaces with little or no boiler shut down. It would also be desirable if the slag that did form could be removed without excessive tube erosion, which can be caused by chemical reactions with iron in the tubes as well as the mechanical efforts to remove tough slag deposits.
Over several recent years the literature has extensively reported that chloride induced corrosion of high temperature surfaces in waste to energy boilers is one of the most costly problems in the industry. This problem can result in replacement of superheater pendants as often as every eight months in some units or the costly use of higher alloyed materials to either shield the metal surfaces or serve as replacement tube material.
The cost-effectiveness of the replacement alloys has not been proven in many cases, and the industry has been looking for alternative solutions. There is a need for chemical solutions to the problem of corrosion in boilers of all types and especially in the high temperature flue gas near WTE superheater pendants.
The problem of corrosion is not limited to WTE boilers. In U.S. Pat. No. 6,478,948, Breen, et al., indicate that until recently, furnace boiler tubes corroded slowly and had a service life of 20 to 30 years, but the introduction of low NOx burners has increased the rate of boiler tube corrosion and can reduce their life expectancy to only 1 to 2 years. Breen, et al., point out that the corrosion of furnace wall tubes involves several mechanisms. First, they say that removal of oxide film from the tubes eliminates protection from the oxide layer and allows further oxidation. Second, they say that if the oxide film is not present, the iron surface is attacked and pitted by condensed phase chlorides which may be present. They also point to a third mechanism which occurs when wet slag runs across the surface of the film. As that happens, iron from the tube goes into the slag solution which contains low fusion calcium-iron-silicate eutectics that are formed in the liquid slag under reducing conditions in the furnace. They state that reduced sulfur in the form of S, H2S, FeS or FeS2 can react with the oxygen of the tube scale depriving the tube metal of its protective layer.
Corrosion can be especially severe in WTE boilers, with areas operating at temperatures within the range of from about 250° C. to about 550° C., such as super heater pendant tubes, can particularly troublesome and costly. While the problems of boiler corrosion are well documented and there is a growing understanding of the causes, the available solutions to these problems are not as easily facilitated or economical as would be desired. In a 2004 paper delivered at NAWTEC, Ken Robbins of Maine Recovery Company detailed attempts to use shielding, alternate metallurgies, and various soot blowing strategies to mitigate corrosion found in a WTE unit. The paper also discussed a proprietary chemical slag control program, which was found helpful in controlling slag and minimizing cleaning outages, but had no discernable effect on specific localized corrosion problems. In the case of isolated corrosion, especially on superheater pendant surfaces, which can experience corrosion rates ranging from 0.020 to 0.050 inches per month, tube failures can occur in as little as seven months and create a need for replacement of the entire pendant annually.
A TNO (Nederlandse Organisatie Toegepast-Voor Naturwetenschappelijk) report entitled “Review on Corrosion in Waste Incinerators and Possible Effect of Bromine” provides a mechanistic explanation for the severe corrosion suffered by WTE units. See Ir. P. Rademakers (TNO IND), Ing. W. Hesseling (TNO-MEP), Ir. J. van de Wetering (Akzo Nobel AMC) (July 2002). In addition to the overall analysis of the primary chemical components involved in this corrosion mechanism, it provides a series of equations that may explain why chloride corrosion occurs at the temperature and metallurgical conditions of a waste incinerator.
It is well known that corrosion by high CO levels and reducing atmospheres occurs in the first pass above the grate in-furnace. A refractory lining is often employed on the water walls in the first pass. A strong temperature gradient and condensing substances can also contribute to reducing conditions in these areas. Alkali metal chlorides have been found in deposits near the metal surface, and the high level of chlorides in the waste are strongly implicated with the problem.
Rademakers, et al., explain that high temperature corrosion in waste incinerators is caused by chlorine either in the form of HCl, Cl2, or combined with Na, K, Zn, Pb, Sn and other elements. Both gaseous HCl with and without a reducing atmosphere and molten chlorides within the deposit, are considered major factors. As with Breen, et al, they point out that sulfur compounds can be corrosive compounds under some circumstances and can influence the corrosion by chlorine.
Rademakers, et al., identify several factors as the most important in high temperature corrosion: the metal temperature and the temperature difference between gas and metal, the flue gas composition, deposits formation and reducing conditions, and the ratio of SO2/HCl. They indicate that following mechanisms can be distinguished:                Corrosion by HCl/Cl2 or SO2/SO3 containing gas under oxidizing or oxidizing/reducing conditions, and        Corrosion by solid or molten deposits of metal chlorides and sulfates.Rademakers, et al., describe these mechanisms and refer to a schematic, in FIG. 1, as drawn from Krause, 1986, 1993, and as set out below in various steps.        
Corrosion caused by chlorine-containing gas at metal temperatures above about 450° C. is referred to as ‘active oxidation’. Alkali chlorides, such as NaCl, CaCl2 and KCl, can be present already or can be formed by the combustion and subsequent reaction of alkali oxides:Na2O+2HCl=2NaCl+H2O  [1]Under ideal conditions (good mixing, sufficient residence time) alkali chlorides can be sulfated according to the following reaction, provided there is enough SO2 and O2:2NaCl+SO2+½O2+H2O=Na2SO4+2HCl  [2]This would result in formation of sulfates and volatile HCl. At the relatively low tube wall temperatures of most waste incinerators, the sulfates are not very harmful and the HCl formed will be transported to the flue gas clean up system. However, if the gas reaches the cooler tube walls before the reaction is completed, the alkali metals will tend to condense on the cooler metal. In this case, further sulfate formation can occur on the metal under the release of HCl, and that causes high chlorine partial pressures and enhanced corrosion.
Without SO2 at 500° C., NaCl and iron oxides can form Cl2:2NaCl+Fe2O3+½O2=Na2Fe2O4+Cl2.  [3]6NaCl+2Fe3O4+2O2=3Na2Fe2O4+3Cl2  [4]Calculations of the dissociation constant of HCl as a function of temperature indicate that chlorine is present as Cl2 under oxidizing conditions up to gas temperatures of 600° C., whereas above 600° C. formation of HCl is enhanced in the presence of water vapor according to the reaction:H2O+Cl2=2HCl+½O2  [5]
Rademakers, et al., state that at about 500° C., Cl2 can penetrate pores or cracks in an oxide layer. At the low oxygen partial pressures that exist near the metal-oxide scale boundary, the metal chlorides are the more stable phase. Reactions 3 and 4 can result in a Cl2 partial pressure sufficiently high that it reacts directly with the steel to form FeCl2:Fe+Cl2=FeCl2 (solid)  [6]The vapor pressures of metal chlorides will depend primarily on the temperature and the HCl content of the gas. In addition, the type of oxide (and alloy) can considerably influence the vapor pressure. The vapor pressure of FeCl2 is already relatively high at low temperatures. As a result, formation of FeCl2 can decrease the adherence of the oxide scale or can cause spallation of the oxide layer.
Rademakers, et al., explain that iron chlorides form and migrate out from the corrosion product due to their volatility. At higher oxygen partial pressures near the oxide-gas interface, these chlorides are then converted to oxides and liberate chlorine. These new oxides are not formed as a perfect layer and do not offer protection. Part of the liberated chlorine migrates back through the oxide/deposit to react with the metal at the oxide-metal interface, and form metal chlorides again:FeCl2 (solid)=FeCl2 (gas)  [7]4FeCl2+3O2=Fe2O3+2Cl2  [8]3FeCl2+2O2=Fe3O4+3Cl2  [9]In this process, the chlorine has a catalytic effect on the oxidation of the metal resulting in enhanced corrosion.
The kinetics of active oxidation is mainly determined by the evaporation and outward diffusion of FeCl2. Similar chlorine corrosion and regeneration cycles may proceed via FeCl3 and it is possible for the ferrous iron to be oxidized to the ferric state, which liberates chlorine when oxidized.4FeCl2+4HCl+O2=4FeCl3+2H2O  [10]4FeCl3+3O2=2Fe2O3+4Cl2  [11]
The volatility of different compounds can be compared based on the temperature T4 (temperature at which the vapor pressure reaches 10−4 bar), and vapor pressure values for some compounds are given in Table 1.
TABLE 1T4 Temperatures of Metal Chlorides of Main Alloying ElementsMetal chlorideT4 (° C.)FeCl2536FeCl3167CrCl2741CrCl3611NiCl2607
From the above, Rademakers, et al., conclude that low alloy steels and iron-base alloys have limited resistance against active oxidation. High alloyed materials, nickel base alloys in particular, have a much better resistance, which may be because chlorides are more difficult to form and, once formed, have a relatively low volatility. Except for the FeCl3, most T4 temperatures are well above 500° C. indicating that this mechanism is most relevant to superheaters and less to evaporators.
Corrosion of heat transfer surfaces in boilers has been a major problem, particularly WTE units which generate highly corrosive flue gases, and continues to trouble the industry. It would be desirable to have a technology that could mitigate corrosion and slag at the same time.
There remains a present challenge to provide a process for taking necessary corrective action to address the slag in boilers, particularly in WTE units, to reduce or eliminate down time for cleaning while also treating the corrosion before damage becomes excessive and requires expensive repair and shut down.